Process for removal of emission gas pollutants by biological means

ABSTRACT

Method for scrubbing gases containing oxygen, that are polluted by biodegradable substances, by bringing them into contact with an aqueous suspension of microorganisms such that: 
     (a) the period of contact between gas and suspension is 0.5-5 seconds; 
     (b) the suspension is brought into contact with the gas again after a retention time of 3-10 minutes has been attained; 
     (c) the suspension is continuously supplied with active biomass in amounts between 0.5% and 5% of the aqueous suspension whereby 
     (d) the excess suspension is drawn off.

The present invention relates to a method for scrubbing emission gasesin which the gases that contain noxious substances and oxygen arebrought into contact with an aqueous suspension of microorganisms withthe result that the noxious substances are removed.

Such methods have already been described. However, the concentration ofpollutants in exhaust gases is usually so low that only a lowconcentration of biomass can be maintained in the aqueous suspension.The known methods of this type have a capability for degradation whichis limited by the low concentration of microorganisms and thereforerequire large volumes of aqueous microorganism suspension and a highrate of circulation in order to achieve a satisfactory removal ofpollutants.

The object of the present invention is to achieve a high rate ofdegradation during relatively short contact periods between gas and thesuspension of microorganisms which has a high concentration ofmicroorganisms and, in relation to the volume of gas to be scrubbed, alow aqueous volume. More precisely, the present invention relates to amethod for scrubbing oxygen-containing gases that are contaminated withbiologically degradable substances by bringing the gases into contactwith an aqueous suspension of suitable microorganisms and which ischaracterised by the following features:

(a) the period of contact between gas and suspension is between 0.5seconds and 5 seconds;

(b) the suspension is brought into contact with the gas again after aretention time of between 3 minutes and 10 minutes;

(c) there is a constant feeding in of active biomass suspension inamounts lying between 0.5% and 5% of the total suspension, whereby

(d) excess suspension is removed.

The continuous introduction of active biomass into the circulatingsuspension between the gas contact phase and the retention time phasemeans that it is possible to maintain a highly active biomass in thesuspension that exhibits a high rate of degradation in the presence oflow concentrations of pollutants, i.e. low concentrations of nutrients.

The contact between gas and liquid is achieved at a ratio of gas volumeflow, under normal conditions, to fluid volume flow of between 6 and 30whereby a large surface area for the transfer of gas into the liquid isachieved by the simultaneous generation of droplets.

The preferred apparatus for the generation of gas-fluid contact is thejet washer or the Venturi washer. The noxious substances enter intosolution at the surface of the liquid droplets during the phase ofcontact between gas and liquid. The highly active biomass results in acontinuous degradation which in turn means that a concentration gradientis maintained at the surface of the liquid and thus inhibits theformation of saturation concentrations in the boundary layer.

This especially high, short-term activity of the biomass, which is ofimportance during the period of contact between gas and suspension, isattained during the retention period following contact, because it isduring this period that a complete degradation of pollutants is carriedout while no further noxious substances are being introduced.

The suspension, which is more or less free of noxious substances, isthen brought into contact with the gas again. This more or less completedegradation of pollutants during the retention period is achieved bypreventing the remixing of the suspension as much as possible before itis brought into contact with the gas again.

In this respect it has been found that in general a two-phase retentionperiod is sufficient. The concentration of biomass in the suspension ispreferably 0.2-1 g dry weight per liter of suspension and in particular0.5-0.8 g dry weight per liter.

The concentration of the active biomass which is being continuouslyintroduced into the system is preferably 0.5-2 g dry weight per literand in particular above 0.8 g dry weight per liter. Concentrations of 1g dry weight per liter are generally sufficient.

The active biomass that, according to this invention, is introduced intothe system is produced in a fermenter as appropriate. The fermenter isoperated as an air-lift fermenter whereby the air that is introducedinto the column of the fermenter contains the emission gas components ata high concentration or substances that are similar to the components ofthe emission gas with respect to their biodegradability. The longerperiod of contact between air and the aqueous suspension in thefermenter (air-lift column) means that a high concentration of biomasscan be maintained with an adequate supply of nutrients.

It is usually necessary to regulate the typical biochemical parameterssuch as pH stabilisation, temperature stabilisation, nutrient supply andremoval of metabolites. According to the invention a maintained pH ofbetween 7 and 8 is preferred. This can be attained by addition of sodiumcarbonate or bicarbonate. The temperature in the fermenter and also inthe suspension that comes into contact with the emission gas should bebetween 25° C. and 35° C., preferably about 30° C.

With the method described in this invention it has been found onlysparingly degradable substances, such as aliphatic and aromaticchlorinated hydrocarbons, can be removed from emission gases by up toone third. If the system is run in two stages, in other words twoscrubbing systems as described in this invention are connected inseries, the level of degradation of these sparingly degradablesubstances is increased to 50%.

The method according to the invention has been found to be especiallyadvantageous because it is relatively insensitive to sudden increases inlevels of acid or alkaline gases. This is because the pH regulationsystem responds quickly. The method according to the invention istherefore particularly suitable as the first stage in a multi-stage gasscrubbing plant whereby the subsequent stage can consist of, forexample, a trickle filter filled with activated charcoal. Such tricklefilters by themselves are very sensitive to variations in pH andtemperature. If the emission gas treatment according to the invention isput in line in front of the trickle filter, the buffer effect of thispreceding wash will synergistically improve the performance of thetrickle filter, especially in long term operation.

The following is a more detailed explanation of the inventionillustrated in the form of an example:

FIG. 1 shows the plan of the example system:

The jet washer 1 with the jet sprays 2 is fed with emission gas viasupply line 3. Pump 7 pumps the aqueous suspension of microorganismsinto the jet washer via supply line 4.

The suspension of microorganisms collects on the floor 5 of the jetwasher after contact with the emission gas. The second retention stagefor the microorganism suspension, which is designed to inhibit remixingof the suspension, is labelled 6. There is a concomitant, continuoussupply of active biomass from fermenter 9 via supply line 13 into theretention time chamber. The fermenter 9 is aerated via supply line 12whereby the aerated gas is supplemented via supply line 11 with gaseousnutrients which have biodegradation properties similar to, and which arepresent in similar amounts to, the noxious subatnces present in theemission gases. Inorganic salt buffer solution is supplied to the filtervia line 19. Excess aqueous solution is removed via line 18. Thescrubbed exhaust gas leaves the filter via line 20. A more detaileddescription of the conditions follows:

The fermenter 9 had a volume of 301 where 21 of nutrient salt solutioncontaining

300 mg (NH₄)₂ HPO

200 mg Mg SO₄

32 mg KCl

30 mg CaCl₂

1000 mg NaHCO₃

were added per hour.

The addition of 250 liters gas per hour to the fermenter was made vialine 12 whereby

3.3 ml/hr methylene chloride

1.25 ml/hr toluene

0.83 ml/hr xylene (technical grade) and

0.41 ml/hr chlorobenzene

were added to the air via line 11.

Biomass suspension containing 1 g dry weight per liter was removed fromthe fermenter at a rate of 2 liter per hour. 1.4 m³ suspension/hour wascirculated through the jet washer 1 via line 4.

The volume of suspension in the base 5 of the jet washer and in theretention time chamber 6 of the jet washer was 801. 10 m³ of air perhour was added via 3. The exhaust gas was then led into a trickle filtercolumn of 300 mm diameter and a working height of 780 mm which contained20 kg granulated activated charcoal.

The content of pollutants in the input emission gas, the content at theoutlet of the jet washer and of the trickle filter, as well as the totalreduction in the amount of pollutants in the scrubbed emission gas aredescribed in table 1 for three different flow rates.

                                      TABLE 1                                     __________________________________________________________________________    (Data expressed as mg/m.sup.3)                                                             Outlet                                                                             Outlet       Outlet                                                                             Outlet       Outlet                                                                             Outlet                               Jet  Trickle                                                                           Total    Jet  Trickle                                                                           Total    Jet  Trickle                                                                           Total                         Inlet                                                                            Washer                                                                             Filter                                                                            reduction                                                                          Inlet                                                                             Washer                                                                             Filter                                                                            reduction                                                                          Inlet                                                                             Washer                                                                             Filter                                                                            reduction           Gas Flow Rate                                                                           12 m.sup.3 /h    5 m.sup.3 /h      3 m.sup.3 /h                     __________________________________________________________________________    Methyl Chloride                                                                         21 15.5 9   57%  21  17   6   71%  17.5                                                                              13.5 3   83%                 Ethyl Chloride                                                                          78 67   35.5                                                                              54%  71.5                                                                              62.5 27  62%  6.6 5    2   70%                 Dichloromethane                                                                         120                                                                              97.5 49.5                                                                              59%  132.5                                                                             120  50.5                                                                              62%  155 140  66.5                                                                              57%                 Dichloroethane                                                                          140                                                                              86   22.5                                                                              84%  90.5                                                                              60   18.5                                                                              77%  110 79   31  72%                 Dimethylether                                                                           83 53   28.5                                                                              65%  61.5                                                                              43   20.5                                                                              67%  64  47   22.5                                                                              65%                 Benzene    2 2    <1  50%  1   <1   <1  100% 1   1    <1                      Toluene   22 18.5 2   91%  15.5                                                                              13.5 <2  87%  7   6.5  <2  71%                 Xylene    84 60.5 <2  98%  75  60   <2  97%  45  25   <2  96%                 Ethylbenzene                                                                            33 25.5 <2  94%  21.5                                                                              16.5 <2  91%  14.5                                                                              9    <2  86%                 Chlorobenzene                                                                           15 10.5 <2  87%  8.5 7    <2  76%  3.5 3.5  2   43%                 Chlorotoluene                                                                           22 13   <2  91%  17  13.5 <2  88%  8   5    <2  75%                 Sum of hydrocarbons                                                                     294                                                                              207.5                                                                              45  85%  249 201.5                                                                              39  84%  165.5                                                                             117.5                                                                              27  84%                 based on propane                                                              __________________________________________________________________________

What is claimed is:
 1. Method for scrubbing gases containing oxygen,that are polluted by biodegradable substances, by bringing them intocontact with an aqueous suspension of microorganisms such that:(a) theperiod of contact between gas and suspension is 0.5-5 seconds; (b) thesuspension is brought into contact with the gas again after a retentiontime of 3-10 minutes has been attained; (c) the suspension iscontinuously supplied with active biomass in amounts between 0.5% and 5%of the aqueous suspension whereby (d) the excess suspension is drawnoff.
 2. Method according to claim 1, whereby the concentration ofbiomass in the aqueous suspension is 0.2-1 g dry weight/liter andpreferably 0.5-8 g dry weight per liter.
 3. Method according to claim 1whereby the inflowing active biomass has a concentration of 0.5-2 g dryweight/liter.
 4. Method according to one of the claim 1 whereby theinflowing biomass is cultured in a fermenter with noxious substanceconcentrations reflecting the content of substances in the gas. 5.Method according to one of the claim 1 whereby a jet washer or a Venturiwasher is used to bring about contact between gas and suspension. 6.Method according to one of the claim 1 whereby a complete remixing ofthe suspension is avoided by means of a two or more stage process ofsuspension retention during the retention period before the suspensionis brought into contact with the gas again.
 7. Method according to oneof the claim 1 whereby the emission gas is subjected to a second phaseof biological scrubbing after it has been in contact with thesuspension.
 8. Method according to claim 7 whereby the second biologicalscrubbing is carried out in a trickle filter containing granulatedactivated charcoal.